S(−)-nicotine (NIC) activates presynaptic and postsynaptic neuronal nicotinic receptors that evoke the release of neurotransmitters from presynaptic terminals and that modulate the depolarization state of the postsynaptic neuronal membrane, respectively. Thus, nicotine produces its effect by binding to a family of ligand-gated ion channels, stimulated by acetylcholine (ACh) or nicotine which causes the ion channel to open and cations to flux with a resulting rapid (millisecond) depolarization of the target cell.
Neuronal nicotinic receptors are composed of two types of subunits, α and β, and assemble as heteromeric receptors with the general stoichiometry of 2α and 3β or as homomeric receptors with 5α subunits. Nine subtypes of the a subunit (α2 to α10) and three subtypes of the β unit (β2 to (β4) are found in the central nervous system. The most common nicotinic receptor subtype in the brain is composed of two α4 and three β2 subunits, i.e., α4β2. These subunits display different, but overlapping, patterns of expression in the brain. Examples of heteromeric receptor subtypes include α4β2, α3β2, α3β4, α6β2, α4α5β2, α6α5β2, α4α6β2, α4β2β4, α3β2β4, and others. The predominant homomeric subtype includes α7, but other combinations have also been proposed.
For the most part, the actual subunit compositions and stoichiometries of nicotinic receptors in the brain remain to be elucidated. Thus, neuronal nicotinic receptor subtype diversity originates from differences in the amino acid sequence at the subunit level and from the multiple combinations of assemblies of subunits into functional receptor proteins, which affords a wide diversity of pharmacological specificity.
In spite of the extensive diversity in neuronal nicotinic receptor messenger RNA expression, only a limited number of tools are available to study the pharmacology of native receptors. Radioligands are used in many studies. [3H]NIC appears to label the same sites in the brain as [3H]ACh. It has been estimated that over 90% of [3H]NIC binding in the brain is due to association with the heteromeric receptor that is composed of α4 and β2 subunits. Also abundant in the central nervous system are the homomeric receptors labeled by [3H]methyllycaconitine (MLA), which has high affinity for the α7 nicotinic receptor subtype. Nicotinic receptor subtypes can be studied using functional assays, such as NIC-evoked neurotransmitter release (e.g., [3H]dopamine (DA) release, [3H]norepinephrine (NE) release, [3H]serotonin (5-HT) release, [3H]gamma-aminobutyric acid (GABA) release and [3H]glutamate release) from superfused rat brain slices. Nicotinic receptors are located in the cell body and terminal areas of these neurotransmitter systems. NIC facilitates neurotransmitter release from nerve terminals.
The structural and functional diversity of central nervous system nicotinic receptors has stimulated a great deal of interest in developing novel, subtype-selective agonists and/or antagonists. Some of these agonists are currently being evaluated in clinical trials for cognitive enhancement and neuroprotective effects, potentially beneficial for disease states such as Alzheimer's and Parkinson's disease.